A gear shaper is a machine tool including a cutter reciprocating mechanism in which a working tool (hereinafter called cutter) is reciprocated in a center axis direction of an object to be machined (hereinafter called a workpiece) so as to machine the workpiece in its advancing path and to be moved away from the workpiece in its returning path. Examples of such a gear shaper are disclosed in Patent Documents 1 and 2 below.
FIG. 7 is a schematic view showing how a workpiece is machined in a conventional gear shaper.
As shown in FIG. 7, for example, the gear shaper as disclosed in Patent Document 1 below performs gear cutting by shaping a workpiece 50 in such a manner that a cutter 13 attached to a main spindle 12 is given a rotational movement together with the workpiece 50 while being reciprocated vertically (arrow L1 in FIG. 7), the workpiece 50 being placed on a table (not shown) on which the workpiece 50 is attached for the machining.
The cutter 13 is in a vertical state near its rising end ([a] in FIG. 7). The cutter 13 is moved straight downward in this state and machines the workpiece 50 during the downward movement ([a] to [c] in FIG. 7). Near its falling end, the cutter 13 is moved away (relieved) R from the workpiece 50 in the arrow L2 direction by the amount of retraction (the amount of relieving) to become in a returning state ([d] in FIG. 7).
Next, the cutter 13 is moved upward in the state of [d]. Near its rising end, the cutter 13 approaches the workpiece 50 by the amount of relieving to become in the vertical state ([a] in FIG. 7). Then, the cutter 13 is moved downward again. By repeating these operations, the cutter 13 machines the workpiece 50 while being reciprocated vertically.
Here, the relieving direction of the cutter in relieving will be described.
FIG. 8 is a schematic view showing the relieving direction of the cutter in relieving.
As shown in FIG. 8, cutting blades 13a are formed in a side surface of the cutter 13. The tip of each cutting blade 13a is located on an addendum circle 13b, and the root of each cutting blade 13a is located on a dedendum circle 13c. 
FIG. 9 is a schematic view showing how an internal gear is formed by the cutter.
As shown in FIG. 9, teeth 50a are formed in an inner surface of the workpiece 50 by the cutter 13. Here, a portion indicated by a broken line in the workpiece 50 is a portion 50d to be chipped.
As shown in FIG. 8, the tip of each formed tooth 50a is located on an addendum circle 50b, and the root of each tooth 50a is located on a dedendum circle 50c. In relieving R, the cutter 13 is relieved R in a direction connecting the center O2 of the cutter 13 and the center O1 of the workpiece 50. In other words, a direction from the center O2 of the cutter 13 toward the center O1 of the workpiece 50 is a relieving direction Rd.
However, in the conventional gear shaper described above, the cutter 13 and the workpiece 50 during machining sometimes interfere with each other (hereinafter called relieving interference) in the relieving R of the cutter 13, depending on the specifications of the workpiece to be machined into an internal gear, the specifications of the cutter, machining conditions, and the like.
Here, the relieving interference will be described.
FIG. 10 is a schematic view showing how the relieving interference occurs.
As shown in FIG. 10, the workpiece 50 during machining includes a portion 50e chipped this time and a portion 50f chipped in a previous rotation of the workpiece 50. For example, at a portion indicated by I in FIG. 10, the cutter 13 in the relieving R as shown by a dashed-two dotted line in FIG. 10 sometimes interferes with the portion 50f chipped in the previous rotation of the workpiece 50.
Note that, in FIG. 10, the relieving interference portion I is briefly illustrated for the sake of understanding; however, the relieving interference actually occurs at multiple positions in an extremely small area between the cutter 13 and the workpiece 50 in FIG. 10 due to machining conditions such as the amount of cut, a cutting speed, and a feed speed. Moreover, if a rotation direction C2 of the cutter 13 and a rotation direction C1 of the workpiece 50 during machining are as shown in the drawing, the interference does not occur in a lower half of the drawing but occurs in an upper half thereof.